Electronic velocimeter with radio frequency coupled coils have been used in the past to measure projectile muzzle velocity, particularly on rapid fire or moving gun systems. The differential coil configuration, as opposed to the single coil design, has been proven to reduce the effect of other gun projectile related parameters and to improve the accuracy of the velocity measurement. The problem with the aforementioned devices, including the device described in U.S. Pat. No. 4228397, has been the rapid deterioration of the coil sensors accuracy due to the severe environment at the muzzle end of the gun. Propellant gases cause the inner loop of the sensor to erode with use. The rate of erosion varies, depending on the type gun and the propellants used. Erosion of the inner loop of the differential sensor causes an imbalance in the differential configuration which induces errors in measurement. In addition when the erosion continues for a sufficient period of time high pressure propellant gases leak between the sensor assembly and the muzzle face and can cause the sensor to be blown off of the gun.
Prior art methods used to measure projectile velocity sometime measure the time taken for a projectile to traverse a known distance between two sensing devices. From this time measurement and known distance, the velocity can be computed. This velocity, however is the average velocity at the midpoint of the reference distance and not the velocity at the muzzle. The aforementioned methods utilize velocity coils, light screens, break wires and strain patches.
Other prior art devices make use of a known time interval and measure the distance the projectile travels in that time. An example of this is the use of high speed photography which uses either visible light or X-rays. The problem with this method is that the velocity is measured at the instrument and not at the muzzle.
Micro-wave interferometery is also used to measure projectile velocity in-bore as well as down range. However, even though this method can provide direct muzzle velocity measurement, the measurement is frequently unreliable because of severe loss of signal due to ionized gases in the muzzle region.
The problem with prior art electronic velocimeters using radio frequency coupled inductive sensor coils, was that even though this method provided a real time measurement of projectile velocity directly at the muzzle, by utilizing only a single sensor at the muzzle and measuring the time interval of the projectile length to pass the sensor, the sensor was rapidly destroyed because of the exposure to the extremely harsh environment.